pathd: fix 'no msd' command not possible under pcc node

pathd: fix 'no msd' command not possible under pcc node

It should be possible to reset the configured msd value,
and not mentioning the previous msd value.

> ubuntu2204(config-sr-te-pcep-pcc)# no msd
> % Command incomplete: no msd
> ubuntu2204(config-sr-te-pcep-pcc)#

Fix this by defining the msd parameter optional, and
separating the 'no msd' command from the 'msd' command.

Fixe: efba0985fcdd ("pathd: Add optional support for PCEP to pathd")

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>